The present invention relates to electroless gold plating compositions. More particularly, the present invention is directed to autocatalytic gold plating compositions useful for plating gold on gold and on nickel.
Electroless plating is useful in applications requiring coatings for complex shapes This capacity makes electroless plating techniques particularly useful in the electronics industry, for example, in the metallization of conductors and insulators in printed circuit boards. Base metals such as nickel and copper are often used in electroless plating processes to metallize conductors and insulators However, because of its low contact resistance and beneficial effects on bonding leads, electroless gold plating is continually desired for coating complex shapes and electrically isolated tracks and bonding pads in printed circuit boards.
It has been found, however, that the stability and plating rate of a particular electroless plating bath is critically affected by the substrate being plated. Plating baths presently used to plate gold onto gold are virtually incapable of plating gold onto nickel and baths used for plating gold onto nickel are extremely unsuitable for plating gold onto gold.
For example, U.S. Pat. No. 4,863,766 to Iacovangelo et al. discloses an electroless plating bath for plating gold onto nickel wherein the bath contains potassium gold cyanide, a water-soluble alkali metal cyanide as a complexing agent, and hydrazine as the reducing agent. The Iacovangelo bath is stable and provides high plating rates while plating gold onto nickel, but plating continues therein only as long as the nickel substrate remains in contact with the plating bath. Plating essentially stops once the nickel substrate is covered with the gold. The oxidation of the reducing agent used in the Iacovangelo bath, hydrazine, is very poorly catalyzed by gold. Furthermore, cyanide, which is used to stabilize the gold complex, i.e., the potassium gold cyanide, is a poison for hydrazine oxidation. The already poor catalytic activity of gold for hydrazine oxidation becomes nil with only trace amounts of cyanide. Because hydrazine is not oxidized on gold in the presence of cyanide, the Iacovangelo bath is not autocatalytic, i.e., it will not plate gold onto gold. Thus, at the present time, separate plating baths are used when it is desired to plate gold onto nickel and then onto the gold deposited on the nickel.
As indicated above, the term "autocatalytic" is used herein to describe a plating system which is capable of depositing gold on a gold substrate. Autocatalytic gold plating is advantageous, for example, for increasing the thickness of existing gold surfaces which are too thin for some uses. Autocatalytic electroless gold plating compositions are known in the art. Reference may be made, for example, to U.S. Pat. Nos. 3,700,469; 3,917,885; and 4,337,091.
U.S. Pat. No. 3,700,469 to Okinaka discloses an autocatalytic electroless gold plating bath containing a soluble gold cyanide complex ion, excess free cyanide to stabilize the gold cyanide complex ion, an alkaline agent as a pH adjustor, and an alkali metal borohydride or dimethylamine borane as a reducing agent. Although a truly autocatalytic plating bath, the Okinaka bath has been found to be extremely sensitive to nickel ions in solution. The use of the Okinaka bath to plate gold directly onto nickel results in a buildup of nickel in the bath to the point wherein the bath shuts down. In addition to nickel sensitivity, the Okinaka formulation also suffers from several limitations which reduce its usefulness for plating gold onto gold. These limitations include instability, low plating rate (about 1 micrometer per hour), plating rate deterioration, and difficulty of bath replenishment.
U.S. Pat. No. 3,917,885 to Baker discloses an autocatalytic electroless plating bath wherein the source of gold is an alkali metal imide complex of the gold rather than an alkali metal gold cyanide. However, the Baker bath, like the Okinaka bath, has been found to be extremely sensitive to the presence of nickel, which results in the shutdown of the plating bath. The Baker bath also shares the other limitations of the Okinaka bath, particularly deteriorating plating rate.
U.S. Pat. No. 4,337,091 to El-Shazly et al. discloses the use of trivalent gold metal complexes as the source of gold in an electroless gold plating bath, the reducing agent being any of the borohydrides, cyanoborohydrides or amine boranes that are soluble and stable in aqueous solution. A later version of the El-Shazly bath, disclosed in U.K. Patent Application No. G.B. Z1Z1444A, uses a mixture of trivalent and monovalent water-soluble gold cyanide complexes. The El-Shazly baths also suffer from high sensitivity to nickel ions and the other limitations found in the Okinaka and Baker baths.
Autocatalytic electroless plating baths having improved stability, higher plating rates, ease of bath replenishment, and decreased sensitivity to nickel ions were achieved in commonly assigned and copending application serial nos. (07/431,360, filed on Nov. 3, 1989, and 07/431,358, filed on Nov. 3, 1989, both to Iacovangelo. The baths described in these references contain a water-soluble alkali metal monovalent gold cyanide complex, a water-soluble cyanide compound as a complexing agent for the gold complex, a carbonate compound, and a reducing agent selected from the group consisting of water soluble alkali metal borohydrides and water-soluble amine boranes, the pH of the plating composition being in the range of about 10 to about 14. The bath described in application Ser. No. 07/431,358, filed on Nov. 3, 1989, referred to above, further contains an aliphatic amine to improve the bath's throwing power, i.e., the ability of the bath to provide a uniform deposit on the entire surface of a substrate. Although the baths described in the Iacovangelo applications have improved stability, increased plating rate, and lower sensitivity to nickel ions, and are extremely effective in plating gold onto gold, there is room for improvement in the ability of these baths to plate gold onto nickel.
As noted previously herein, separate baths are currently required when it is desired to plate gold first on a nickel substrate and then on the gold deposited on the nickel substrate. It would be desirable to provide a single electroless plating bath which is capable of effectively plating both gold and nickel substrates.